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• Wastewater characteristics
• Overview of wastewater treatment processes
• Wastewater treatment processes
– Pretreatment: bar racks, grit removal, flow
equalization
– Primary treatment
– Secondary treatment
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• Suspended solids: can cause sludge deposits and
anaerobic conditions in the environment
• Biodegradable organics: can cause anaerobic
conditions in the environment
• Pathogens: transmit disease
• Nutrients: can cause eutrophication
• Heavy metals: toxicity to biota and humans
• Refractory organics: toxicity to biota and humans
• Dissolved solids: interfere with reuse, damage the
aquatic ecosystem
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5
• Significant variation in wastewater characteristics
• Some industry may generate certain toxic pollutants
Industry Pollutant Concentration
(mg/L)
Coke byproduct (steel mill)1 Organic nitrogen
Phenol
100
2000
Metal plating2 Chromium (VI) 3-550
Plywood glue waste3 COD
Phenol
2000
200-2000
1코크스제조부산물(제강공정), 2금속도금, 3합판접착제폐액
• May need pretreatment to prevent high loading to
the wastewater treatment plant
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• Major goal: remove BOD and suspended solids (SS)
– US (1970’s): “fishable and swimmable water”, 30/30 rule
– Currently we are also concerned with protecting aquatic
ecosystem
– Current Korean standard: BOD ≤ 10 mg/L, SS ≤ 10 mg/L
• Additional goal: remove N, P, pathogens, total
dissolved solids (TDS), trace pollutants (heavy metals,
EDCs, PPCPs, …)
7
Trea
tme
nt
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• Pretreatment: removes materials that
can cause operational problems,
equalization optional
• Primary treatment: remove ~60% of SS
and ~35% of BOD
• Secondary treatment – remove ~85% of
BOD and SS
• Advanced (tertiary) treatment –
additional removal of BOD & SS, N & P,
and others
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• Goal: to remove large objects that would damage
or foul pumps, valves, and other mechanical
equipment
http://www.infobarscreens.comProvided by McGraw-Hill
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• Goal: remove “grit”
• Grit
– Inert dense material such as sand, broken glass, silt, and
pebbles
– Abrades pumps and other
mechanical devices in the
following treatment
processes
– Relatively high density �
settles down easily “Vortex type” grit chamber
http://www.hatchmott.com
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• The flow rate and strength of wastewater
varies from hour to hour
• High flow rate and strength in the morning,
low at night; high flow rate and low strength
during storm events
• Flow equalization is to achieve nearly constant
wastewater flow rate and strength � better
performance of wastewater treatment and
reduce the size and cost
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• Removal of suspended solids by settling
• This removes some BOD as well!
• Removes ~60% of SS and ~35% of BOD
• Sludge (= mass of settled solids) is produced
• Floating materials such as oil and grease are also
removed
• Design parameters (recall water treatment!):
retention time (~2 hr), overflow rate
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• Rectangular or circular
http://www.mlive.com http://www.lgam.info
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• Goal: provide BOD removal beyond what is
achieved in primary treatment
– Removal of soluble BOD
– Additional removal of SS
• How??
– Use microorganisms to convert organic wastes into
stabilized compounds
� Bring the naturally-occurring process (recall self-
purification in streams) into an engineering setting, but
creating more favorable condition such that things can occur
much faster!
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• Availability of high density of microorganisms
• Good contact between organisms and wastes
• Favorable temperature, pH, nutrients, carbon
source (food)
• Oxygen (or other electron acceptors)
• No or little toxic chemicals present
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• Dispersed (suspended) growth
– Activated sludge
– Oxidation ponds
• Fixed (attached) growth
– Trickling filters
– Rotating biological contactors (RBCs)
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http://www.phlush.org http://www.niwa.co.nz
• Activated sludge– Most common
– Large plants
• Oxidation pond– For small communities
– Low energy & maintenance cost
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http://www.purewatergazette.net http://www.sswm.info
• Trickling filter– Water trickles through the
porous media
• Rotating biological contactor
– The water gets oxygen when
exposed to the air
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• Monod equation
� = 1����� ���� =
����� + � = �� ��� + �
μ = specific growth rate (d-1)
X = biomass concentration (mg/L)
μm = maximum specific growth rate (d-1)
S = food (substrate) concentration (mg/L)
Ks = half saturation constant (mg/L)
“saturation type” kinetics
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0
2.5
5
0 20 40 60 80 100 120
00
��2
��
�� 2�� 3�� 4�� 5�� 6��
• S>>Ks: μ = μm
(maximum
growth rate); the
growth rate is
independent of S
• S<<Ks: limited
food supply; the
growth rate is
proportional to S
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• Overall change in biomass concentration
– Microorganisms not only grow, but also die or get eaten! (this is called as “decay”)
– So:
(net biomass growth rate)
= (rate of growth by substrate utilization) – (rate of decay)
�� = ���� =
����� + �� − ���
rg = net biomass growth rate (mg/L/d)
kd = decay rate constant (d-1)
growth decay
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• Substrate utilization (=degradation) rate
– Microorganisms utilize food (substrate) to grow
– A fraction of the utilized substrate is converted to biomass (microorganisms are not 100% efficient!)
��� = −���� =1������ + ��
rsu = substrate utilization rate (mg/L/d)
Y = yield coefficient (mg biomass/mg substrate)
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Textbook Ch 11 p. 519-537